When you mess with the Amazon rainforest you mess with a lot of things — 2.5 million species of insects, 40,000 species of plants, 1,300 species of birds, and those are only the known ones. The 1.4 billion of acres of thriving, sprawling biology that cover the Amazon help drive the very metabolism of a continent. And now it appears that the rainforest is at least partly responsible for something else: the Amazonian clouds themselves. Clear-cut the land and you could, in effect, clear-cut the sky.

That improbable idea comes courtesy of a paper just released in the journal Science, the product of work done by researchers at the Max Planck Institute for Chemistry in Mainz, Germany. The clouds in the Amazon, just like everywhere else, consist of water vapor clinging to tiny clumps of carbon compounds. In forested areas, the carbon compounds are byproducts of plants’ metabolism; in populated areas, they are often from human pollution. Most of the time, atmospheric chemists can see the carbon clumping taking place; when the microscopic bits reach a certain size, they are able to attract and hold water. In the Amazon, the clumps seem to appear out of nowhere, nearly fully formed. No one has ever been able to catch them in the act of coming together.
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Max Planck graduate student Christopher Pohlker traveled to a pristine stretch of forest in Brazil to see if he could solve the riddle. He gathered a bit of rainforest air, using an instrument that sucks a sample through a fine nozzle and sprays it onto a ceramic square half a millimeter on each side, where any microscopic airborne particles get stuck. To figure out the chemical make-up of those particles, he and his colleagues brought the squares to Lawrence Berkeley National Laboratory in California and placed them in the facility’s synchrotron, where X-rays of varying energies were fired at the collected specks. The specific frequencies that were absorbed could reveal the samples’ chemical makeup.

What the researchers found was a mix of carbon compounds, plus one other thing: potassium — and that told them a lot. Potassium salts appear to be good at getting carbon compounds to stick together. The larger a carbon cluster was, the larger the ratio of carbon compounds to potassium within it, suggesting that just a certain amount of potassium was needed to get the accretion process started, and after that the carbon compounds kept piling on of their own accord. That, in turn, would get water droplets forming.
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The real surprise was the source of the potassium. Forest fires often release the element into the air, but there were none burning when Pohlker took the samples. “Since we can rule out the burning source in our samples,” he says, “the other source seemed to be the biosphere itself.” In other words, the forest.

Plants and fungi can release potassium into the air under certain conditions. Fungi in particular are veritable fountains of the stuff: when they shoot out their spores, they also spray out a potassium-rich fluid. Biologists working with leaf molds and other fungi in the lab had noticed this, and atmospheric chemists had noticed that there seemed to be a lot of potassium floating above the Amazon in the wet season. Pohlker’s adviser, chemist Meinrat Andreae, in fact recently reported that a third of the Earth’s land surface is probably covered with microscopic fungi. But until now, no one had linked potassium from fungi to cloud formation. “We think the residue of these droplets is what we are observing,” Pohlker says. “It’s really impressive.”

Pohlker, Andreae, and their colleagues ran the numbers and found that the amount of potassium particles released from microscopic fungi in the lab was indeed enough to account for the concentration of potassium they observed in their samples. But there are still some crucial experiments left to do: specifically, they have not yet actually verified that the microscopic fungi living on the forest trees in the Amazon are in fact releasing the potassium they see in the air. “What we’re still lacking is a demonstration that if you go to a plant in the Amazon and put a plastic bag around it, you’ll see these particles coming off,” Andreae says. “That’s one of the things we want to do next.”

Even when that’s done, it’s not clear everyone will be sold on the new findings — or at least on their thoroughness. “Are these particles only relevant directly over the rain forest, or are they lofted by convection and transported to surrounding regions?” wrote Yale University professor and climate modeler Trude Storelvmo in an email. Yet another topic for future research is the question of whether the Amazon is the only rainforest that gets the potassium cycle going this way or if other — perhaps all — rainforests do it. What’s settled science now, however, is that just as the Amazon is dependent on the rain and sunlight provided by the sky, the sky is dependent on the nourishment from the forest. The circle of life just added another ring.